Hydrogenation of alkene is the addition of H₂ across a C=C double bond in presence of a metal catalyst like Ni, Pd, or Pt, forming an alkane.

1. Based on Substitution of Double Bond

2.Cis vs Trans Alkenes

Reason:

• Cis isomer is less stable (higher energy)
• Therefore hydrogenates faster

3.Cyclic vs Acyclic

cyclopropane reacts faster due to ring strain.

4. Conjugated vs Isolated

Isolated double bond hydrogenates normally.

In conjugated systems, 1,4-addition may occur depending on conditions.

More stable alkene → Lower rate of hydrogenation why?

Hydrogenation is an exothermic reaction.

• More stable alkene = lower potential energy
• Therefore it releases less heat on hydrogenation
• Lower energy difference → smaller driving force
• Hence reacts more slowly

1. Which alkene hydrogenates fastest?

A) CH₃–CH=CH₂
B) CH₂=CH₂
C) (CH₃)₂C=CH₂
D) (CH₃)₂C=C(CH₃)₂

Answer: B

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2. The rate of hydrogenation is inversely proportional to:

A) Heat of hydrogenation
B) Stability of alkene
C) Surface area of catalyst
D) Pressure of H₂

Answer: B

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3. Which has lowest heat of hydrogenation?

A) 1-Butene
B) cis-2-Butene
C) trans-2-Butene
D) 2-Methyl-2-butene

Answer: D

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4. Correct order of rate of hydrogenation:

A) Tetra > Tri > Di > Mono
B) Mono > Di > Tri > Tetra
C) Tri > Di > Mono > Tetra
D) Di > Mono > Tri > Tetra

Answer: B

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5. Which reacts faster in hydrogenation?

A) trans-2-Butene
B) cis-2-Butene
C) Both equal
D) 2-Methyl-2-butene

Answer: B

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6. Hydrogenation of alkene is:

A) Anti addition
B) Syn addition
C) Free radical reaction
D) Electrophilic addition

Answer: B

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7. Which alkene is most stable?

A) CH₂=CH₂
B) CH₃–CH=CH₂
C) (CH₃)₂C=CH₂
D) (CH₃)₂C=C(CH₃)₂

Answer: D

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8. Which will hydrogenate slowest?

A) 1-Butene
B) 2-Butene
C) 2-Methylpropene
D) 2,3-Dimethyl-2-butene

Answer: D

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9. More stable alkene has:

A) Higher heat of hydrogenation
B) Lower heat of hydrogenation
C) Same heat of hydrogenation
D) Zero heat of hydrogenation

Answer: B

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10. Strained cyclic alkene hydrogenates faster because:

A) It is more stable
B) It has lower energy
C) Relief of ring strain
D) It is more substituted

Answer: C

JEE/NEET Concept

Heat of hydrogenation order:$$\text{Cyclopropene > Cyclobutene > Cyclopentene > Cyclohexene}$$

Higher heat release means less stable alkene → faster hydrogenation.

MCQs

1. Which cyclic alkene hydrogenates fastest?

A) Cyclohexene
B) Cyclopentene
C) Cyclobutene
D) Cyclopropene

Answer: D

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2. The fastest hydrogenation is shown by the alkene having:

A) Maximum stability
B) Maximum ring strain
C) Maximum substitution
D) Minimum heat of hydrogenation

Answer: B

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3. Arrange in increasing rate of hydrogenation:

A) Cyclohexene < Cyclopentene < Cyclobutene < Cyclopropene
B) Cyclopropene < Cyclobutene < Cyclopentene < Cyclohexene
C) Cyclobutene < Cyclopropene < Cyclohexene < Cyclopentene
D) Cyclopentene < Cyclohexene < Cyclobutene < Cyclopropene

Answer: A

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4. Cyclohexene hydrogenates slower than cyclobutene because:

A) It is more substituted
B) It has less ring strain
C) It has higher energy
D) It forms unstable product

Answer: B

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5. Heat of hydrogenation is maximum for:

A) Cyclohexene
B) Cyclopentene
C) Cyclobutene
D) Cyclopropene

Answer: D

Numerical-Type Concept Question

The heats of hydrogenation are:

• Cyclohexene → −119 kJ/mol
• Cyclopentene → −120 kJ/mol
• Cyclobutene → −127 kJ/mol
• Cyclopropene → −151 kJ/mol

Question:

Which is most stable? Which reacts fastest?

Solution:

✔ Most stable: Cyclohexene (least heat released)
✔ Fastest reaction: Cyclopropene (highest heat released)

“Hydrogenation rate of cyclic alkenes depends on ring strain. Smaller rings possess greater angle strain and hence higher heat of hydrogenation. Therefore, they react faster.”

JEE/NEET Important Order

Rate of hydrogenation:$$\textbf{Isolated diene > Conjugated diene > Aromatic compound}$$

Comparison Example (Very Important for Exams)

Compare:

1. 1,3-Butadiene (conjugated)
2. 1,4-Pentadiene (isolated)

Stability Order:

$$\textbf{Conjugated > Isolated}$$

Rate of Hydrogenation:

$$\textbf{Isolated > Conjugated}$$

JEE/NEET Important Order

Rate of hydrogenation:$$\textbf{Isolated diene > Conjugated diene > Aromatic compound}$$

(Benzene hydrogenates very slowly due to aromatic stability)